A variety of overload control mechanism is known from the prior art for application in digital mobile communication networks in cases of congestion. One way of “controlling” the load on the communication network which is utilized in the global system for mobile communications (GSM) consist of rejecting communication requests of a mobile station with a message forbidding the mobile station to access the channel for some specified length of time. This mechanism prevents any further repetitions by the mobile station, either through its automatic repetition scheme or through repeated user requests. This mechanism is used in GSM when over load threatens the overall throughput.
Another approach which is utilized in GSM consists in forbidding a whole population of mobile stations to access the cell through an indication on the broadcast control channel (BCCH). This enables a cut-down in the traffic at the source without incurring any additional traffic toward mobile stations. In order to achieve this, subscribers are split into balanced-sub-populations, through a random allocation controlled by their operator.
The access class to which a given subscriber belongs is stored in the subscriber identity module (SIM) and is therefore available to the mobile station. In normal load situations all classes are allowed access. When traffic must be cut down, the base station controller (BSC) can decide to block one, two or any number of these access classes, reducing statistically the amount of traffic by a corresponding percentage.
As in the GSM system the air-interface is also a shared resource in UMTS. As a consequence the UMTS standard allows certain congestion-control mechanisms in order to take the appropriate actions when congestion is detected. Depending on the degree of congestion the consequences can be very varied. Some possible consequences are:                lost calls,        a higher bit error rate (BER) and thus—in the case of voice transmission—bad sound quality,        service refusal.        
One approach for managing a congestion situation which is compliant with the UMTS standard is to decrease the quality of service (QoS) in order to allow more users to be active at the same time. The channel capacity requirement of each ongoing communication session is decreased by allowing an increase of the frame error rote (FER) when congestion is detected and thereby reducing the power with which each user is served—power is the shared resource here.
U.S. Pat. No. 6,167,237 shows a universal wireless communication system comprising at least two wireless subsystems which is operated at the edge of spectral efficiency while at the same time allowing wireless communication stations to operate such that optimal power saving is obtained under acceptable transmission conditions. A radio base station of one of the subsystems measures whether a wireless communication station of another one of the subsystems causes adjacent channel interference in a channel its own subsystem. If so, the radio base station commands the wireless communication station of the other substation to adjust its spectral transmission characteristic such that the transmit spectrum becomes narrower. If no interference is detected the wireless communication station of the other subsystem is allowed to broaden its spectrum so that a less complex transmit signal processing, such as filtering, con be applied and thus power saving is achieved.
U.S. Pat. No. 6,097,700 shows a packet switched radio channel congestion control system for controlling congestion on a packet switched radio channel (PRCH) is disclosed. A PRCH congestion control function determines if the average time delay for packet calls on the PRCH is within a delay alarm level. If the average time delay is not within the delay alarm level, it is then determined which packet calls to expel from the PRCH in order to decrease the average time delay. The determination for congestion may be made for each of the uplink and downlink of a PRCH separately, or, on the combined uplink and downlink of the PRCH. A similar system is known from U.S. Pat. No. 5,757,772.
It is a common disadvantage featured by the above described prior art approaches to handle congestion that the individual user may either experience a temporary discontinuation of the desired communication service or a significant reduction in sound quality and thus intelligibility of a telephone call.